1. Field of the Invention
The present invention relates to a high-dielectric constant metal/ceramic/polymer composite material and a method for producing an embedded capacitor. More specifically, the present invention relates to an embedded capacitor with a high dielectric constant in which ceramic insulating particles and metal particles are simultaneously mixed with a polymer resin so that the ceramic particles are bound to the metal particles.
2. Description of the Related Art
With recent advances in the miniaturization and high-frequency characteristics of multilayer substrates, passive devices mounted on conventional PCB substrates have been obstacles in the miniaturization. Particularly, due to a rapidly increasing tendency toward embedding and an increase in the number of I/O terminals in semiconductors, it is difficult to ensure spaces sufficient to accommodate a number of passive devices around active integrated circuit chips. The driving frequency of semiconductors is increased, which requires the use of decoupling capacitors for stably supplying an electric power supply from input terminals. When such decoupling capacitors are at a close distance from input terminals, inductance arising from high frequency can be reduced. To meet the demand for miniaturization and high-frequency characteristics, some solutions have been proposed to overcome the limitation associated with the arrangement of capacitors around active integrated circuit chips. For example, methods have been proposed to embed chips under active integrated circuit chips of substrates and to reduce the inductance of chips. Various approaches to the proposals have been attempted, e.g., development of low ESL MLCCs.
Conventional high-dielectric constant materials for embedded capacitors are materials in which a ferroelectric powder, which is known to have a high dielectric constant, is dispersed in an epoxy resin. However, since the epoxy resin, acting as a matrix, has a low dielectric constant, capacitors are formed in serial connection despite an increase in the dielectric constant of the powder. Accordingly, the overall dielectric constant of the materials is greatly dependent on the low dielectric constant of the epoxy resin. No particular methods have been suggested to increase the dielectric constant of materials, other than increasing the volume ratio of powder to resins. However, since the volume fraction of powder is limited to a specific level in PCB lamination processes, there is a difficulty in increasing the volume fraction of powder. If the volume fraction of powder is increased, the processing characteristics are inevitably deteriorated. Accordingly, the dielectric constant is substantially unchanged within the volume fraction range applicable to the processes.
In an attempt to overcome these problems, a method for increasing the dielectric constant of a composite material is suggested in Japanese Patent Application No. 2001-68803. According to this method, a material having a relatively high conductivity is added to allow a filler to serve as a conductor, not as a dielectric material, instead of adding a high-dielectric constant material to increase the overall dielectric constant of a composite material.
However, conductive fillers, such as metals, tend to agglomerate, and thus it is difficult to uniformly disperse the fillers in resins. In addition, when a conductive filler having a very high conductivity is added in a powder form, dielectric breakdown may readily occur. Accordingly, when a conductive material is added in a specific volume fraction, percolation, which is a term used in the mixing rule, occurs, and as a result, the conductive material is not used as a dielectric film but as a conductor film. Further, an increase in dielectric loss is induced by an eddy current generating within conductive materials according to the changes in frequency. That is, in the case where a general ferroelectric filler, BaTiO3, is used, the dielectric constant sharply increases in a specific volume ratio. Accordingly, effective control of increased conductivity arising from the addition of metal powder and an increase in the volume fraction of metals to the most extent may be a method for increasing the dielectric constant of composite materials. It is important to create a situation that can prevent the occurrence of percolation despite the addition of a large amount of metal powder.
To this end, U.S. Patent Publication No. 2002-0039667 discloses a method for coating the surface of a metal powder with an insulating dielectric layer. The advantages of this method are that the occurrence of percolation between metal particles is inhibited and the dielectric constant of the dielectric layer is increased.
According to the method, however, space charges are decreased due to coating with the insulating layer, and hence the content of a filler to be percolated is increased. That is, since the filler must be present in an amount of 30% by volume or more to achieve a high dielectric constant, the processability is deteriorated. In addition, since additional processing, such as annealing, must be performed, the procedure is complicated, uniform coating is difficult and sufficient reproducibility cannot be ensured.